The only known antagonist to general anesthesia is hydrostatic pressure. This fact has lead to the concept that general anesthetics act by expanding membranes and pressure opposes their effects by counteracting this expansion. To quantitatively test this hypothesis we are measuring the partial molar volumes of general anesthetics in lipid bilayers. This is done using a small, novel excess volume dilatometer. Using this sensitive instrument we have determined the excess volume of mixing of a number of volatile anesthetics with lipid bilayer suspensions. The partial molar volume of these agents in the lipid bilayer may then be calculated if the lipid to water partition coefficient is known. Three volatile agents have been examined in bilayers of several compositions. In general the partial molar volumes are similar in magnitude to the liquid molar volumes of these agents. From this information we may calculate that at general anesthetic doses the lipid bilayer is expanded by about half a percent.